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Cell nucleus
From : Eukaryotes are s whose cells have a . The nucleus is surrounded by a double membrane (commonly referred to as a nuclear membrane or nuclear envelope), with pores that allow material to move in and out. *They also have a variety of internal membrane-bound structures, called organelles, and a composed of s, s, and s, which play an important role in defining the cell's organization and shape. *Eukaryotic is divided into several linear bundles called s, which are separated by a during nuclear division. Eukaryotic cells are typically much larger than those of s having a volume of around 10,000 times greater than the prokaryotic cell. # # # # # # # # # # # # # Eukaryotic cell nucleus From : In , the nucleus is a found in . The nucleus is the control center of the cell. The cell nucleus contains all of the cell's organized as multiple long linear molecules in a with a large variety of s, such as s, to form s. The main structures making up the nucleus are the , a double membrane that encloses the entire organelle and isolates its contents from the cellular , and the (which includes the ), a network within the nucleus that adds mechanical support, much like the , which supports the cell as a whole. Because the nuclear envelope is impermeable to large molecules, s are required to regulate of molecules across the envelope. The pores cross both nuclear membranes, providing a through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and s. Chromosome From : A chromosome is a deoxyribonucleic acid ( ) molecule with part or all of the genetic material ( ) of an organism. Most chromosomes include which, aided by , bind to and the DNA molecule to prevent it from becoming an unmanageable tangle. Chromosomes in eukaryotes are composed of fiber. Chromatin fiber is made of s ( s with part of a DNA strand attached to and wrapped around it). Chromatin fibers are packaged by proteins into a condensed structure called . Nucleosome From : A nucleosome is a basic unit of packaging in , consisting of a segment of DNA wound in sequence around eight cores. Nucleosomes form the fundamental repeating units of , which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it (in mammalian cells approximately 2 m of linear have to be packed into a of roughly 10 µm diameter). Nucleosomes are thought to carry inherited information in the form of s of their core . The nucleosome core particle consists of approximately 146 s (bp) of wrapped in 1.67 left-handed around a octamer. Core particles are connected by stretches of "linker DNA", which can be up to about 80 bp long. Gene From : A gene is a sequence of in or that for a that has a function. During , the DNA is first . The RNA is the for a that performs a function. The transmission of genes to an organism's is the basis of the inheritance of . Some genetic traits are instantly visible, such as or number of limbs, and some are not, such as , risk for specific diseases, or the thousands of basic processes that constitute . Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas the genomes of complex s, including humans, contain an absolute majority of DNA without an identified function. This DNA has often been referred to as " ". RNA splicing From : RNA splicing, in , is a form of RNA processing in which a newly made (pre- ) is transformed into a ( ). During splicing, s are removed and s are joined together. Splicing takes place within the immediately after . Splicing is usually required in order to create an mRNA molecule that can be . For many eukaryotic introns, splicing is carried out in a series of reactions which are catalyzed by the , a complex of small nuclear ribonucleo proteins ( s). Intron From : An intron is any within a that is removed by during of the final RNA product. The term intron refers to both the DNA sequence within a gene and the corresponding sequence in RNA . RNA splicing follows and precedes . Introns can be located in a wide range of genes, including those that generate s, (rRNA), and (tRNA). Sequences that are joined together in the final mature RNA after RNA splicing are s. Non-coding DNA From : s are non-coding sections of a gene, transcribed into the sequence, but ultimately removed by during the processing to mature . Many introns appear to be . Studies of s from indicate that some introns appear to be selfish genetic elements. It was originally suggested that over 98% of the does not encode protein sequences, including most sequences within s and most , while 20% of a typical genome is noncoding. Some introns appear to have significant biological function, possibly through functionality that may regulate and activity as well as protein-coding gene expression, evident in hosts that have become dependent on such introns over long periods of time. Many noncoding DNA sequences must have some important biological function. This is indicated by studies that report highly , sometimes on time-scales of hundreds of millions of years. This implies that these noncoding regions are under strong ary pressure and . For example, in the genomes of s and , which diverged from a 65–75 million years ago, protein-coding DNA sequences account for only about 20% of conserved DNA, with the remaining 80% of conserved DNA represented in noncoding regions. often identifies chromosomal regions associated with a disease with no evidence of functional coding variants of genes within the region, suggesting that disease-causing genetic variants lie in the noncoding DNA. The significance of noncoding DNA mutations in cancer was explored in April 2013. Ultraconserved regions From Ultraconserved regions (UCRs) are regions over 200 bp in length with 100% identity across species. These unique sequences are mostly found in noncoding regions. It is still not fully understood why the negative selective pressure on these regions is so much stronger than the selection in protein-coding regions. Though these regions can be seen as unique, the distinction between regions with a high degree of sequence conservation and those with perfect sequence conservation is not necessarily one of biological significance. One study in Science found that all extremely conserved noncoding sequences have important regulatory functions regardless of whether the conservation is perfect, making the distinction of ultraconservation appear somewhat arbitrary. Ciliate From : Unlike most other , ciliates have two different sorts of : *A tiny, (the "generative nucleus," which carries the germline of the cell), *A large, (the "vegetative nucleus," which takes care of general cell regulation, expressing the of the organism). The micronucleus passes its genetic material to offspring, but does not express its genes. Periodically the macronuclei must be regenerated from the micronuclei. Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the of the organism. Macronuclear DNA is derived from micronuclear DNA by amazingly extensive DNA rearrangement and amplification. The macronucleus begins as a copy of the micronucleus. The micronuclear chromosomes are fragmented into many smaller pieces and amplified to give many copies. The resulting macronuclear chromosomes often contain only a single gene. In Tetrahymena, the micronucleus has 10 chromosomes (five per haploid genome), while the macronucleus has over 20,000 chromosomes. During development of the macronucleus, numerous "Internal Eliminated Sequences" (IESs) are deleted and the remaining gene segments, Macronuclear Destined Sequences (MDSs), are spliced together to give the operational gene. More than 95% of micronuclear DNA is eliminated during macronuclear development. Paramecium From : Like all ciliates, Paramecium has a dual nuclear apparatus, consisting of a , and one or more . The macronucleus controls non-reproductive cell functions, expressing the genes needed for daily functioning. The micronucleus is the generative, or nucleus, containing the genetic material that is passed along from one generation to the next. Fission may follow , a sexual phenomenon in which Paramecium of compatible mating types fuse temporarily and exchange genetic material. During conjugation, the micronuclei of each conjugant divide by and the gametes pass from one cell to the other. The gametes of each organism then fuse to form micronuclei. The old macronuclei are destroyed, and new ones are developed from the new micronuclei. Macronucleus From : A macronucleus is the larger type of in s. Macronuclei are . The macronucleus contains hundreds to thousands of chromosomes, each present in many copies. During conjugation, the macronucleus disintegrates, and a new macronucleus is formed by of the . Polytene chromosome From : Polytene chromosomes are large s which have thousands of strands. The chromosomal strands are formed after repeated division of the chromosome in the absence of cytoplasmic division. This type of division is called . Polytene chromosomes, at interphase, are seen to have distinct thick and thin banding patterns. The polytene chromosome contains two types of bands, dark bands and interbands. The dark bands are darkly stained and the inter bands are lightly stained with nuclear stains. The dark bands contain more DNA and less RNA. The interbands contain more RNA and less DNA. The amount of DNA in interbands ranges from 0.8 - 25%. Internally eliminated sequences The macronucleus contains no or very little junk dna. From : Internally eliminated sequences (IES) are noncoding regions of the germ-line genome found in Ciliates. They are defined as sections of DNA removed from the diploid micronuclear genome during which a copy of the micronuclear genome is converted to the macronuclear genome. Nucleolus From : The nucleolus is the largest structure in the of . It is the site of . This assembly not only involves the rRNA, but ribosomal proteins as well. The genes encoding these r-proteins are transcribed by pol II in the nucleoplasm by a "conventional" pathway of protein synthesis (transcription, pre-mRNA processing, nuclear export of mature mRNA and translation on cytoplasmic ribosomes). The mature r-proteins are then "imported" back into the nucleus and finally the nucleolus. Association and maturation of rRNA and r-proteins result in the formation of the 40S (small) and 60S (large) subunits of the complete ribosome. These are exported through the nuclear pore complexes to the cytoplasm, where they remain free or become associated with the endoplasmic reticulum, forming rough endoplasmic reticulum (RER). Category:Life